Deuterium NMR Study of Molecular Order and Dynamics in 60CB

Abstract

Abstract To test models for molecular reorientation in liquid crystals, the biphenyl core of a perdeuterated nematic liquid crystal, 4-cyano-4′ −hexyloxy-d13−biphenyl-d8 (60CB-d21) was studied using deuterium NMR spectroscopy. Two overlapped doublets, one from each deuterated phenyl ring, were seen in the deuterium spectrum of 60CB-d21. An order parameter tensor of the molecular core was used to account for these doublet splittings. The asymmetry parameter η of the electric field gradient was found to be different for the deuterons on the two rings. The spin-lattice relaxation times TIZ and TIQ were measured at 15.3 MHz for these rings, using a deconvolution routine that found the areas of two peaks in partially relaxed spectra. Four spectral densities of motion were derived as a function of temperature in the nematic phase. The small step rotational diffusion model or the “third rate anisotropic viscosity” model may be used to account for the measured spectral densities.